FCETAL NUTRITION: THE PLACENTA 489 



membrane, and has lost all, or nearly all, its physiological 

 activity.-^ 



What is the difierence in the early stages of pregnancy, when 

 the trophoblast is morphologically well-developed ? We believe 

 that at that time the extra-embryonic ectoderm has less to do 

 with the quantity, and more with the quahty, of the material 

 transferred to the new organism. It does not act merely by the 

 physical laws of diffusion and osmosis. At this stage the cells of 

 the ovum have not yet departed widely from a general type, and 

 the active trophoblast would seem to spare the embryonic cells 

 much of the work of the elaboration of the food-materials, and 

 thus conserve their energies for their own multipHcation and 

 differentiation. As the cells gradually depart further and in 

 different directions from the original type, each cell requires 

 to expend less energy on its own specialisation ; at the same 

 time the nutritive wants become more varied, and each cell 

 requires to expend more energy on the synthesis of its individual 

 protoplasm. As the duties of selection and anabolism are 

 more and more taken up by the cells themselves, the trophoblast 

 has a less important part to play, and it undergoes a gradual 

 process of degeneration.^ 



' Hofbaner's observations on the haemoglobin metabolism, already quoted 

 (see p. 480), furnish concrete evidence of such a change in the trophoblast. 

 In the first half of pregnancy the syncytium breaks down the maternal 

 haemoglobin, and subsequently b\iilds it up in part for the foetus. But in the 

 second half, though a greater daily supply of organic iron is required for the 

 formation of hsemoglobin and other purposes (see p. 515), the amount of loosely 

 bound iron-compounds in the villi is " extraordinarily small." The only 

 explanation is that the larger molecules of the more firmly combined iron- 

 compounds are not attacked and broken down so strongly by the syncytium, 

 but are passed on to the foetal circulation. 



^ A similar change occurs in the decidual cells of the rabbit. In the first 

 periods of their existence, they synthesise and store a large quantity of 

 glycogen. In the last week, the cells of the foetal liver assume their glyco- 

 genic function, apparently absorbing the carbohydrate from the foetal blood 

 as it returns from the placenta, and the decidual cells degenerate with the 

 loss of their function. 



